We have recently shown that intravenous infusions of the synthetic human parathyroid hormone fragment 1-34 can be conveniently used to stimulate renal secretion of 1,25-(OH)2 vitamin D in humans, and assess their secretory reserve for this important hormone. We have also shown that such secretory reserve is absnet in elderly subjects with idiopathic osteoporosis, even though their serum levels of 1,25-(OH)-2 vitamin D are initially within the normal range. We want to test the physiological significance of this theoretically important discovery, by measuring its effects on intestinal calcium absorption, parathyroid hormone secretion, and bone mineral mobilization, and its correlation with bone mass and reates of bone loss, in elderly people with idiopathic osteoporosis. Normal young adults will be studied for comparison. The long term goal of these studies is to define etiologically homogenous subsets of osteoporotic patients within the population of "senile osteoporosis" patients, to identify those osteoporotic patients likely to benefit form 1,25-(OH)2 vitamin D replacement therapy, and to establish that stimulation tests of 1,25-(OH)2 vitamin D secretion and parathyroid hormone secretion have diagnostic and investigative utility in humans.